1. Field of the Invention
The present invention relates to the wiring layout of an integrated circuit. More particularly, the present invention relates to the wiring layout for the pull-up and pull-down devices of an off-chip driver.
2. Description of the Related Art
Due to the rapid increase in the level of integration, compact and lightweight electronic products have emerged. To catch up with these highly functional products, the layout of a chip carrier for supporting a silicon chip in a package and the layout of a printed circuit board (PCB) for assembling electronic components and other circuit boards have been improved. Because the pin-counts of electronic devices increase with the level of integration, the number of bonding pads on a circuit board must increase correspondingly. Consequently, density of wires on a circuit board must also increase. In addition, due to the high-density layout and miniaturization of signaling lines, distance of separation between neighboring bonding pads must be greatly reduced as well.
FIG. 1 is the layout and wiring connections of pull-up and pull-down devices in a conventional off-chip driver (OCD).
The off-chip driver (OCD) as shown in FIG. 1 includes bonding pads 11 and 12, pull-up drivers 21 and 22, pull-down drivers 23 and 24, pull-up signaling lines 31 and 32 and pull-down signaling lines 33 and 34. The pull-up driver 21 and the pull-down driver 23 provide pull-up voltage and pull-down voltage to the bonding pad 11, respectively. Similarly, the pull-up driver 22 and the pull-down driver 24 provide pull-up voltage and pull-down voltage to the bonding pad 12, respectively. The pull-up driver 21 and the pull-down driver 23 are located on opposite sides (for example, the upper and lower sides as shown in FIG. 1) of the bonding pad 11. The pull-up driver 22 and the pull-down driver 24 are also located on the upper and lower sides of the bonding pad 12, respectively. In other words, both pull-up drivers 21 and 22 are on the same side above the bonding pads 11 and 12 while both pull-down drivers 23 and 24 are on the same side below the bonding pads 11 and 12.
The pull-up signal lines 31 and 32 are connected to the pull-up driver 21 and the pull-up driver 22 through conductive lines 41 and 42, respectively. Hence, a pull-up voltage is provided to each active device 51 inside the pull-up driver 21 and each active device 52 inside the pull-up driver 22. The active devices 51 and 52 are metal-oxide-semiconductor (MOS) transistors, for example. Note that the conductive line 41 branches off from the pull-up signal line 31, runs along the side of the pull-down driver 23, the bonding pad 11 and the pull-up driver 21 and finally enters the pull-up driver 21. The conductive line 42 branches off from the pull-up signal line 32, runs along the side of the pull-up driver 22 and finally enters the pull-up driver 22. The pull-down signal lines 33 and 34 are connected to the pull-down driver 23 and the pull-down driver 24 through conductive lines 43 and 44, respectively. Hence, a pull-down voltage is provided to each active device 53 inside the pull-down driver 23 and each active device 54 inside the pull-down driver 24. Note that the conductive line 44 branches off from the pull-down signal line 34, runs along the side of the pull-up driver 22, the bonding pad 12 and the pull-down driver 24 and finally enters the pull-down driver 24. The conductive line 43 branches off from the pull-up signal line 33, runs along the side of the pull-down driver 23 and finally enters the pull-down driver 23.
The number of bonding pads is not restricted to just two. Obviously, for more than two bonding pads, the increase in the number of pull-up drivers is equal to the number of pull-down drivers.
However, as the number of bonding pads increases, extra space must be set aside for laying the pull-up and pull-down conductive lines for connection with the pull-up and pull-down drivers. Hence, overall layout area will increase.
In addition, as the distance of separation between a pull-up driver or a pull-down driver and bonding pad increases, overall length of the conductive lines leading into the pull-up driver or the pull-down driver also increases. Hence, signal transmission time increases.
Furthermore, as the level of integration of integrated circuits continues to decrease, not only do the dimensions of each electronic device decrease, but area for laying conductive lines shrink as well. Consequently, how to reduce the area necessary for laying connection lines is an important issue in the semiconductor industry.